1. Field
The exemplary embodiments generally relate to hydraulic power units and, more particularly, to controls for hydraulic power units.
2. Brief Description of Related Developments
Generally hydraulic pumps used in mechanized equipment such as, for example, recycling shears and bailers have a higher speed rating than the motors which power the pumps thereby limiting the flow of the pump. To compensate for speed rating of the motor, a fixed volume pump may be coupled with a variable volume pump to obtain a greater flow rate through the hydraulic system.
Generally, the installation of a variable flow and/or fixed volume pump includes a fixed speed electric motor. The controls for the variable volume pump generally include a torque limiter that limits the torque load on the motor. Also known as a constant horsepower control, the torque limiter maximizes the flow output of the pump without overloading the motor. For example, referring to FIG. 1, as the pump pressure increases (i.e. the motor torque needed to pump the fluid increases) the input power needed by the motor also increases. The torque limiter control takes control of the flow when the input power reaches the power rating of the fixed speed motor. The fluid flow is then regulated such that the power required by the motor remains constant as the pressure increases. It is noted that if a variable volume pump is installed with a fixed volume pump, the flow from the variable volume pump may be increased or decreased even though the driving motor remains at a constant or fixed speed.
A typical power unit pump for mechanized equipment may include a torque limited piston pump (variable volume pump) coupled with a fixed volume vane pump (or gear pump). Generally, both of the pumps are driven by a fixed speed electric motor. Referring to FIG. 2 as the fluid pressure increases in this typical pump-motor group the power required from the motor also increases. When the motor is loaded to its power rating, any further fluid pressure increase would overload the motor. Generally, the fixed volume pump is vented out of the hydraulic circuit and its flow returns directly back to a fluid reservoir of the hydraulic system. The power required by the motor drops as a result of the flow from the fixed volume pump being directed directly back to the reservoir. As the fluid pressure continues to increase (through work of only the piston pump) the power required by the motor again reaches the power rating of the motor. The torque limiting control for the piston pump causes the displacement of the piston pump to decrease thereby reducing the flow from the piston pump. It is noted that the above power unit pump has a flow output through the pumps that is limited by the lower rated speed of the motors powering the pumps.
Further, conventional hydraulic pump and motor systems remain running even when the machine they are integrated into is idle. Generally the motors in these systems have restrictions as to how many times the motor may be started and stopped within a predetermined time period.
It would be advantageous to be able to use pumps in a hydraulic system at their rated speed capacity where the speed rating for the accompanying motor is rated less than the speed of the pump.